The present invention relates to a tuning circuit of an electronic tuning type receiver. More particularly, the invention relates to a tuning circuit in which the fluctuation of a resonance frequency caused by an input signal is suppressed by connecting a capacitor having a predetermined capacitance to each of two variable capacitance diodes connected in series-opposition.
Heretofore, a tuning circuit as shown in FIG. 1 is known in which variable capacitance diodes (hereinafter referred to as "Varicap diodes") C.sub.v1 and C.sub.v2 are connected in series-opposition and a tuning capacitor C and a tuning coil L are connected in parallel to the serially connected Varicap diodes. A d.c. bias voltage for tuning is applied to a junction point A of the Varicap diodes C.sub.v1 and C.sub.v2.
When a signal voltage is applied to the tuning coil L, the capacitances of the Varicap diodes C.sub.v1 and C.sub.v2 vary inversely with one another. That is, if the capacitance of the Varicap diode C.sub.v1 increases, the capacitance of the Varicap diode C.sub.v2 decreases, and vice versa. This is due to the fact that the signal voltages applied to the Varicap diodes C.sub.v1 and C.sub.v2 are opposite in phase in each other.
If a signal voltage E.sub.in is applied to the tuning coil L, the capacitances C.sub.1 and C.sub.2 of the Varicap diodes C.sub.v1 and C.sub.v2 can be expressed by the following expressions. When E.sub.in &gt;0 (see FIG. 2), EQU C.sub.1 =C.sub.v1 +C.sub..DELTA.vs1 EQU C.sub.2 =C.sub.v2 -C.sub..DELTA.vs2,
when E.sub.in &lt;0, EQU C.sub.1 =C.sub.v1 -C.sub..DELTA.vs1 EQU C.sub.2 =C.sub.v2 +C.sub..DELTA.vs2
where C.sub.v1 and C.sub.v2 are the capacitances of the Varicap diodes at a predetermined reference voltage, e.g. zero volts, and C.sub..DELTA.vs1 and C.sub..DELTA.vs2 are the capacitance variations of the Varicap diodes C.sub.v1 and C.sub.v2.
The voltage variation at the junction point A when the signal voltage E.sub.in is applied can be expressed as follows: ##EQU1## Since C.sub..DELTA.vs1 &gt;C.sub..DELTA.vs2 (as shown in FIG. 2), ##EQU2## On the other hand, when E.sub.in &lt;0, ##EQU3## In this case, however, since C.sub..DELTA.vs1 &lt;C.sub..DELTA.vs2, the following inequality can be established taking E.sub.in &lt;0 into account. ##EQU4##
The above expressions or inequalities imply that when the signal voltage is applied, the voltage at the junction point A shifts in the positive direction as indicated by the dotted line in FIG. 4, resulting in a variation in the resonance frequency.